3GPP specifies AMR and AMR-WB as mandatory speech codecs for voice services in 3 G networks. These codecs are also mandatory for 3GPP VoIP service that is specified within the 3GPP multimedia telephony via IMS. The ruling specification for the media handling and interaction is 3GPP TS 26.114. Despite the mandatory status of these codecs there is presently the desire in 3GPP to specify new voice codecs that will enable even higher service quality than what is possible with AMR-WB.
However, introducing a new speech codec into a speech communications system may be problematic in some respects. One problem is that there is always an installed base of legacy equipment (both terminals and network infrastructure) that does only support the existing 3GPP codecs or just one of them, for instance AMR-WB, rather than the new codec. This may lead to interoperability problems in which communication between new and legacy equipment is not possible unless proper mechanisms are implemented in the system. Traditional ways to address this problem is the provisioning of transcoders in e.g. media gateways that translate between the new and the old coding formats, or the provisioning of the legacy codecs besides the new codec in new terminals that allows choosing the legacy coding format when a connection to a legacy terminal is established. This latter method requires that there is a capability exchange between the terminals prior to the actual speech connection that identifies the common codec that both terminals support. Within the IMS the session description protocol (SDP) IETF RFC 4566 is used to carry out this capability exchange.
The above described ways for ensuring interoperability when introducing a new codec into a communication system are not the only possibilities and have various disadvantages. The provisioning of transcoders means additional equipment that raises the network investment and maintenance costs. Transcoding is also associated with undesirable speech quality degradations. Using the capability exchange between the terminals prior to the call is a very elegant way, which however may not always be possible. Examples where this is not always possible are multi-party conferencing, hand-over scenarios with mobile users roaming to cells without MTSI support, voice messaging. Also from terminal implementation point of view it may be undesirable to provide support for the complete set of new and legacy codecs as this may increase implementation and technology licensing costs.
Consequently, there is a need for enabling the introduction of new speech codecs into telecommunication systems to provide an improved quality of service, in particular to 3GPP systems, whilst maintaining backwards compatibility with old or legacy codecs.